1. Field of the Invention
The invention relates to the process of manufacturing an integrated circuit. More specifically, the invention relates to a method and an apparatus for using pupil filtering to mitigate optical proximity effects that arise during an optical lithography process used in manufacturing an integrated circuit.
2. Related Art
Recent advances in integrated circuit technology have largely been accomplished by decreasing the feature size of circuit elements on a semiconductor chip. As the feature size of these circuit elements continues to decrease, circuit designers are forced to deal with problems that arise during the optical lithography processes that are used to manufacture integrated circuits. This optical lithography process generally begins with the formation of a photoresist layer on the surface of a semiconductor wafer. A mask composed of opaque regions, which are generally formed of chrome, and light-transmissive clear regions, which are generally formed of quartz, is then positioned over this photo resist layer coated wafer. (Note that the term “mask” as used in this specification is meant to include the term “reticle.”) Light is then shone on the mask from a visible light source or an ultraviolet light source.
This light is generally reduced and focused through an optical system that contains a number of lenses, filters and mirrors. The light passes through the clear regions of the mask and exposes the underlying photoresist layer. At the same time, the light is blocked by opaque regions of mask, leaving underlying portions of the photoresist layer unexposed.
The exposed photoresist layer is then developed, typically through chemical removal of the exposed/non-exposed regions of the photoresist layer. The end result is a semiconductor wafer with a photoresist layer having a desired pattern. This pattern can then be used for etching/implanting on underlying regions of the wafer.
Phase shifters are often incorporated into a mask in order to achieve line widths that are smaller than the wavelength of the light that is used to expose the photoresist layer through the mask. During phase shifting, destructive interference caused by two adjacent clear areas on a mask is used to create an unexposed area on the photoresist layer. This is accomplished by exploiting the fact that light passing through a mask's clear regions exhibits a wave characteristic having a phase that is a function of the distance the light travels through the mask material. By placing two clear areas adjacent to each other on the mask, one of thickness t1 and the other of thickness t2, one can obtain a desired unexposed area on the underlying photoresist layer caused by interference. By varying the thickness ti and t2 appropriately, the light exiting the material of thickness t2 is 180 degrees out of phase with the light exiting the material of thickness t1. Phase shifting is described in more detail in U.S. Pat. No. 5,858,580, entitled “Phase Shifting Circuit Manufacture Method and Apparatus,” by inventors Yao-Ting Wang and Yagyensh C. Pati, filed Sep. 17, 1997 and issued Jan. 12, 1999.
For example, FIG. 1 illustrates how a phase shifter comprised of a zero-degree clear region (phase shifting region 104) and a 180-degree clear region (phase shifting region 106) separated by a chromium regulator 105 is used to achieve a smaller line width for a gate region of a transistor. Note that FIG. 1 does not show masks, but rather a composite layout showing the resultant design and the phase shift areas. In order to produce these features, two masks are generally used: one dark field mask with the phase shifters; and the other a complementary trim mask with protection for regions defined by the phase shifting mask and to define cross-hatched areas. Furthermore, note that as circuit dimensions on semiconductor chips become progressively smaller, phase shifters can be used to define other critical-dimension features in addition to transistor gates.
Unfortunately, the printed image that results from the above-described process is subject to proximity effects that can cause edges of features to deviate from desired locations. Some of these proximity effects are caused by the optical system that is used to expose the photoresist layer.
Note that in order to take advantage of phase shifting masks, coherent illumination (with low σ) needs to be used. This coherent illumination is the reason for large proximity effects (see [Pierrat 2000] “Investigation of Proximity Effects in Alternating Aperture Phase Shifting Masks”, by C. Pierrat, 20th Annual BACUS Symposium on Photomask Technology, 13-25 September 2000, Monterey, Calif.).
For σ=0, the image on the wafer is equal to the convolution of the mask amplitude by the point-spread function of the optics. In other words, the spatial frequency spectrum of the mask is multiplied by the pupil function of the optics to obtain the spatial frequency spectrum of on the wafer.
FIG. 2A illustrates an idealized “pupil function” for an optical system that uses a coherent light source to project an image of the mask onto the photoresist layer. Because of physical limitations of the optical system, this pupil function cuts off all spatial frequency components greater than NA/λ or less than −NA/λ, where NA is the numerical aperture of the optical system, and where λ is the wavelength of the coherent light source.
The resulting point-spread function for this pupil function (illustrated in FIG. 2B) can be determined by taking the Fourier transform of the pupil function. This point-spread function illustrates how the image of one point is spread by the optical system. Note that ringing arises in the point-spread function because the pupil functions sharply cuts off higher frequency components. This ringing a major factor in causing undesired optical proximity effects (see [Pierrat 2000]).
These optical proximity effects can be corrected, or at least compensated for, by adjusting the width of the chromium regulator 105 between phase shifting regions 104 and 106. However, as integration densities continue to decrease, even reducing the width of the chromium regulator to zero may not provide sufficient correction (see [Pierrat 2000]).
Proximity effects can also be corrected by varying the shifter width. However, shifter width cannot be made too small because of mask fabrication issues (resolution and overlay) and wafer process latitude issues. Moreover, the shifter width cannot be made too large without increasing the risk of phase conflicts. See also U.S. patent application Ser. No. 10/082,697 filed Feb. 25, 2002, entitled “Optical Proximity Correction For Phase Shifting Photolithographic Masks”, having inventors Christophe Pierrat and Michel Côté and assigned to the assignee of this application, for a discussion of OPC techniques for phase shifting masks.
Proximity effects can also be corrected by adding assist features (e.g., hammerheads or serifs) to shapes within a layout to compensate for optical effects.
Note that the correction techniques described above rely on modifying the mask layout. The main problems with these techniques is either: (1) their efficacy (chrome regulator width); (2) their practicality (shifter width); or (3) their complexity from a mask and layout standpoint (assist features).
Hence, what is needed is a method and an apparatus for correcting optical proximity effects that addresses the root cause of the optical proximity effects—the sharp cut-off of the pupil function—without the problems of the above-described techniques.